Add support for a target rid

[pombredanne]

It could be useful to also support resolution for a target runtime id, though a NuGet is not runtime-specific as far as I can see.

Note that while this may work for the modern project-reference-based projects, this may not work as expected with the legacy project-reference or the packages.config-based project where the APIs do not seem to have provisions for rid support. Rid support may also require the actual download of the nupkg or extra APIs calls.